My first impression is that it takes a long time to complete its matching and uploading. I'm at over 24 hours now, and there's still a lot of uploading of artwork and remaining songs to go. At least twice, I think iTunes just stopped the step three. At least there was no indication of activity on the screen. So I clicked Start again and it started from scratch. Steps 1 and 2 go quickly, though, and the remaining items to upload keeps getting smaller.

I mostly listen to podcasts or iTunes U lectures or audiobooks, but occasionally music is nice. But hand selecting music results in a pretty small selection, and once you've heard the first few songs in a playlist, starting it over is pretty boring. Hence iTunes Match.

It turns out that we may also gain a good deal from the way iTunes Match will allow you to download a high-quality version of a song you had a low-quality version of--if it's matched. This doesn't work with songs iTunes cannot match and are just uploaded.

Finally, I found one site that claimed that in iOS 6, iTunes Match will stream a song by default, but also allow you to download it to your iPhone so you can listen while offline.

By the way, you can disable cell streaming if you want, and iTunes Match will then only stream or download songs when you're connected to WiFi. That could save some bucks if you're on a limited-data plan (which I will be next week).

"Design is not just what it looks like and feels like. Design is how it works."
Steve Jobs

I have now tried the Macworld tip and it seems to work well. I found I had more than 2,000 tracks that were eligible for iTunes Match downloads. I'm replacing them with the higher-quality tracks a batch at a time. Many were tracks I'd ripped from CDs we own, at lower quality AAC. Others were older tracks.

[09 22 12 Additional]
I've used both methods in the links above, and both work fine. Note that even using an external editor like Sound Studio didn't allow me to "increase" the bit rate of some tracks. (Note that increasing the bit rate is not intended to make the track sound better, just to allow iTunes Match to see if it has a match.)

"Design is not just what it looks like and feels like. Design is how it works."
Steve Jobs